Data, digital or coaxial transmission cables for transmitting data and digital signals into homes have been used for many years and are usually buried underground, often beneath pavements and the front gardens of the homes to which data is being supplied. There are various different cables commonly in use, but a typical cable 1 is shown in FIG. 1 and generally comprises a core 2 formed from a bundle of conductors 3 (only four of which are shown in FIG. 1). Each conductor 3 may comprise a number of stranded copper core filaments 4 twisted together and covered by an insulating polymer sheath 5. A plurality of conductors 3 may be twisted or otherwise bundled together to form the core 2. The conductor bundle forming the core 2 is received in an outer sheath 6 to keep the conductor bundle together and to provide overall mechanical, weather, chemical and electrical protection. The sheath 6 can be formed from aluminium, lead or steel and may have a polymer coating. A filler material 7 is received between the outer sheath 6 and the core 2 and surrounds and fills the interstices of the core 2 formed by the conductor bundle. This filler material 7 can be formed from polymer and is usually soft, although it is sometimes formed from a relatively hard polymeric material. Alternatively, it can sometimes be oil based. The filler material 7 protects and supports the core 2 and can act to seal any nicks or cuts in the outer sheath 6 should they occur, thereby preventing moisture ingress which may lead to consequential failure of the cable 1.
Some cables 1 are also provided with an envelope 8 which surrounds the core 2 and the filler material 7 within the outer sheath 6. This envelope 8 may wind helically around the core 2 and filler material 7 and overlap so as to completely surround or wrap the filler material 7. The envelope 8 is commonly formed from paper, cellophane or similar material, but can also be formed from aluminum foil.
In another, unillustrated, cable type, the envelope is embedded within the filler material so that the filler is between the core and the envelope and between the envelope and the inner surface of the outer sheath. Some cables may also have a further, external envelope (not shown) which surrounds the filler material, in addition to the envelope that is embedded within the filler material. The external envelope separates the filler material that is on the outside of the envelope from the inner surface of the outer sheath. The external envelope may be formed from cellophane, with the internal envelope formed from aluminium or paper.
Developments in technology, together with demand for better and faster data transfer, have recently necessitated the use of optical fibres for data transmission purposes or at least a hybrid core comprising both traditional copper wire and optical fibres, as optical fibre is capable of transmitting much larger quantities of data at high speed relative to traditional copper cored cables. Where optical fibre is required, the standard approach is to disconnect the existing cable and to lay an entirely new cable containing the optical fibre, with the old disconnected copper cored cable either being removed altogether or left in the ground. It will be appreciated that this approach necessitates the digging of a trench in order to lay the new cable, which is disruptive, time consuming and expensive.
U.S. Pat. No. 7,814,654B2 discloses a method for removing a cable core from an outer cable sheath so that the optical fibre can be fed through the vacated outer cable sheath without having to dig a trench. In the method described in this document, a flowable medium is introduced under pressure into an annular space between the core and the sheath, so as to reduce friction between the core and the sheath to the extent necessary to allow the core to be drawn out of the sheath for subsequent insertion of the new optical fibre. However, this method is troublesome as it is difficult to pump the fluid into the annular space between the core and the sheath, especially for cable runs of any length. Relatively high pressures are required to ensure that the fluid has passed along the cable and completely surrounds the core to an extent sufficient to reduce the friction between the core and the sheath and thereby enable the core to be extracted without a high degree of difficulty. The high pressures required may also damage the outer sheath making it impossible to re-use.